PET is produced via an esterification reaction of the carboxyl groups of terephthalic acid and the hydroxyl groups of ethylene glycol, wherein water separates. The process waste water accumulating through the esterification reaction, which in addition to acetaldehyde, i.a., contains considerable amounts of ethylene glycol, is subjected to a rectification (process column) for the purpose of the recovery of the ethylene glycol in the sump of the column. The ethylene glycol may then either be recovered or returned to the process for preparing PET. In order to keep the loss of ethylene glycol low, methods according to the prior art as a rule have structurally sophisticated rectification columns with a plurality of plates or packings. A relatively high reflux ratio is necessary for a satisfactory separation of the contaminants contained in the process waste water, which means a substantial power requirement. Despite the separation process that is expensive in terms of energy, the partially purified process wastewater obtained is usually subsequently guided over a stripper due to its residual content of acetaldehyde and other volatile organic components before it is fed to a water treatment plant. In the PET production process moreover substantial amounts of contaminated ethylene glycol (spent ethylene glycol, SEG) accumulates as a byproduct of the polycondensation reaction. The SEG comes primarily from the vacuum system and the ethylene glycol circuit of prepolycondensation and contains 2-methyl-1,3-dioxolane (MDO), in which ethylene glycol is bonded in the form of an acetal. The SEG process flows from the various process steps of the PET production process are subjected jointly with the process wastewater to a reprocessing in the rectification column (also referred to here as “process column” or “column”) and then fed again to the PET production process. The MDO is a low-boiler, which cannot be held back in the process column, i.e., leaves the process column overhead. Most of the MDO is then condensed and reaches the process wastewater as a contaminant. A small part of the MDO is emitted via the exhaust air. The MDO is the actual reason for the relatively high losses of ethylene glycol in the process column of PET systems.
International patent application WO 96/35654 discloses a method in which the process wastewater from the PET production process after the process column (overhead product) is partially condensed and subsequently subjected to an inert gas stripping of a reverse osmosis. A reconcentration of the ethylene glycol remaining in the partially purified process wastewater is thereby carried out through the application of a pressure that is greater than the osmotic pressure of the reconcentrated solution. The water is thus pressed through the membrane against the tendency towards the osmotic concentration equilibrium. The permeate comprises pure water, which can be directly discharged so that a (complex) water treatment plant is no longer necessary. In addition, the method permits higher ethylene glycol concentrations in the overhead product of the process column, which on the one hand reduces the thermal energy required for the column and as a result reduces the coolant required for the subsequent condensation. Furthermore, the number of the column trays required can be reduced. The retentate (concentrate) from the reverse osmosis is returned to the process, wherein a recirculation is provided either into the reactor or directly into the process column. Through this method, although the recovery of ethylene glycol by means of reverse osmosis is improved compared to conventional methods, the problem of the loss of ethylene glycol in the form of MDO is neither recognized nor solved.